1. Field of the Invention
The present invention is concerned with a driving device for driving a pickup arm in a linear tracking pickup apparatus which is used for the reproduction of recorded sound or video of a phonograph disc or video disc.
2. Brief Description of the Prior Art
As a pickup device or apparatus which completely eliminates the development of lateral tracking errors and inside forces, there has been proposed and put into practice the so-called linear tracking pickup device or apparatus.
In such a linear tracking pickup device, the pickup arm is adapted to be moved linearly radially of a phonographic disc or video disc while being held tangential relative to the signal-carrying groove of a phonographic disc or video disc. Accordingly, the sensor lever of the pickup head which is mounted at the foremost end of the pickup arm assumes a position which is tangential relative to the disc as in the case of a disc cutting machine. Since the virbration axis of the pickup head is in agreement with the vibration axis of the groove, the signal which has been recorded on this groove can be faithfully reproduced and the damage of the groove can be minimized.
The greatest technical difficulty in this known linear tracking pickup device or apparatus is found in how to construct the driving device for moving the pickup arm toward the center of the disc progressively in accordance with the turning of the disc while keeping this arm tangential relative to the signal-carrying groove of the disc. An arrangement that the pickup arm is merely held for movement radially of the disc and that the arm is moved directly by the force applied to the stylus of the pickup head from one side of the wall surfaces of the signal-carrying groove of the disc in accordance with the turning of the disc cannot be adopted because an unduly great pressure is applied to one side of the wall surfaces of the groove.
Thus, the pickup arm driving device of this type is, in general, has a motor exclusively for driving a pickup arm, and is operative so that the pickup arm is adapted to swing in the direction of the center of the disc; that, whenever the pickup arm is offset for a trifle angle .DELTA..theta. from the tangential direction of the signal-carrying groove of the disc and whenever any tracking error develops, the motive force of the arm drive motor is transmitted to a supporting member which supports the base end of the pickup arm to move this supporting member linearly radially of the disc for such a distance as is necessary for cancelling the offset angle .DELTA..theta..
However, many of the known pickup arm driving devices are comprised of the so-called mechanical servo mechanism in which the offset angle .DELTA..theta. of the pickup arm is purely mechanically detected and the motive power of the arm drive motor is also mechanically transmitted to the supporting member which supports the base end of the pickup arm. Thus, the driving device of the pickup arm is quite complicated in its arrangement.
Since a pickup arm forms, together with a pickup head, a vibration system, it will be understood that the coupling of a mechanical servo mechanism having such a complicated arrangement and such a great mechanical inertia as mentioned above to a pickup arm is not desirable because this coupling will tend to adversely affect the sound characteristics and to develop noises, deteriorating a signal-noise ratio of such a pickup apparatus.
Also, a pickup arm driving device is required to satisfy a number of driving modes which include: in addition to automatic feeding of the pickup arm at the time of reproduction of the signal recorded on a disc, (a) stopping of feed of the pickup arm in the midst of this reproduction; (b) manually controlled feeding of the pickup arm to an arbitrary position on the disc; and (c) automatic or manually controlled quick returning of the pickup arm upon completion of the reproduction.
In the aforesaid mechanical servo mechanism, however, the structural components thereof are mechanically coupled to each other, and the mechanical inertia of the mechanism is great. Thus, a great difficulty is encountered in materializing a pickup arm driving device which can satisfy such various driving modes as discussed above. For these reasons, in some of the known pickup arm driving devices, there are adopted desperate countermeasures of, for example, additionally providing an electric motor exclusively for performing quick return of the pickup arm. Thus, the structure of the driving device is all the more complicated and increases in size.